The conventional closed-loop mixture control system using an exhaust composition sensor is limited in performance by the accuracy of the signal provided by the composition sensor. If the sensor should fail, it is likely that the output signal of the sensor has a value which clamps the control loop at an air-fuel ratio so that the engine operates too lean under particular conditions, with the consequent loss of engine output power. From the standpoint of vehicle safety, it is desirable for the control loop to be clamped at a rich mixture rather than at a lean mixture in order to avoid the loss of available engine output power during an emergency.